1. Technical Field
The present disclosure relates to a radar device.
2. Description of the Related Art
Recent studies have been made on a radar device using a radar transmission signal of a short wavelength including microwave or milliwave with which a high resolution is achieved. Development is required on a radar device (wide-angle radar device) that detects objects (targets) including a pedestrian as well as a vehicle in a wide-angle range in order to improve outdoor safety.
A known example of such a radar device is a pulse radar device that repeatedly emits pulsed waves. A wide-angle pulse radar that detects a vehicle and a pedestrian in a wide-angle range receives a mixture of multiple reflected waves from a target (for example, a vehicle) at a short distance and a target (for example, a pedestrian) at a long distance. This requires (1) a radar transmitter to have a configuration to transmit pulsed waves or pulse-modulated waves having a auto-correlation characteristic (hereinafter, referred to as a low range sidelobe characteristic) that achieves low range sidelobes, and (2) a radar receiver to have a configuration with a wide reception dynamic range.
Examples of the configuration of the wide-angle radar device include the following two configurations.
The first configuration transmits pulsed waves or modulated waves as radar waves by mechanical or electrical scanning using a directional beam of a narrow angle (beam width of a few degrees), and receives reflected waves using a narrow-angle directional beam. With this configuration, the scanning needs to be performed a large number of times to obtain a high resolution, which leads to a degradation in the performance of following a fast moving target.
The second configuration uses a method (direction of arrival (DOA) estimation) of receiving reflected waves through an array antenna including multiple antennas (antenna elements), and estimating the arrival angle of the reflected waves using a signal processing algorithm based on a reception phase difference corresponding to an antenna spacing. This configuration allows the radar receiver to estimate the arrival angle even when a frequency of scanning of a transmission beam on the radar transmitter is reduced, thereby achieving a shortened scanning time and an improved following performance as compared to the first configuration. Examples of DOA estimation methods include a Fourier transform based on matrix calculation, a Capon method and a linear prediction (LP) method based on inverse matrix calculation, and a multiple signal classification (MUSIC) and an estimation of signal parameters via rotational invariance techniques (ESPRIT) based on eigenvalue calculation.
Disclosed is a radar device (also referred to as a MIMO radar) that includes multiple antennas (array antennas) on the radar transmitter as well as the radar receiver and performs beam scanning by signal processing using transmitting and receiving array antennas (see Jian Li, Stoica, Petre, “MIMO Radar with Colocated Antennas,” Signal Processing Magazine, IEEE Vol. 24, Issue: 5, pp. 106-114, 2007, for example).
In order to achieve a high directional gain of an array antenna, antenna elements (hereinafter, referred to as array elements) included in the array antenna are each formed of a subarray antenna including multiple antenna elements in some cases.
As for the element spacing of the array antenna, it is difficult to arrange the array elements at spacings smaller than the size of the array element. However, the dimension of the array element having a subarray antenna configuration is large, and accordingly a large spacing is needed between subarray antennas, which may generate a grating lobe on a directivity pattern of the array antenna.